1. Field of the Invention
The invention relates to a technical field of a constant-temperature type crystal oscillator (hereinafter called constant-temperature type oscillator) using chip resistors as heating elements, and in particular, to a constant-temperature type oscillator in which the heat conduction property to the crystal unit is enhanced.
2. Description of the Related Art
Constant-temperature type oscillators keep the operational temperatures of their crystal units constant. Therefore, the constant-temperature type oscillators are especially adopted for communication devices for a base station with high frequency stability of, for example, 0.1 ppm or more. JP-A-2005-341191 discloses one example of the constant-temperature type oscillator, for in which downsizing has been attempted by use of chip resistors as heating elements.
FIGS. 4A to 4C are diagrams for explanation of a related art constant-temperature type oscillator. FIG. 4A is a cross-sectional view of the related art constant-temperature type oscillator, FIG. 4B is a view of an oscillator output circuit thereof, and FIG. 4C is a view of a temperature control circuit thereof.
The constant-temperature type oscillator shown in FIGS. 4A to 4C includes an oscillator output circuit 2 including a crystal unit 1 and a temperature control circuit 3. In the constant-temperature type oscillator, respective circuit elements 4 are installed on a first circuit substrate 5a and a second circuit substrate 5b along with the crystal unit 1. Then, the constant-temperature type oscillator is configured to house those in an oscillator case 6. The crystal unit 1 has a crystal element 1a which is formed as, for example, an AT-cut crystal element or an SC-cut crystal element. The crystal element 1a is electrically and mechanically connected to a pair of lead wires 1c of a metal base 1b, and is hermetically encapsulated with a metal cover 1d. 
The crystal unit 1 has a frequency-temperature characteristic that approximately 80° C. at the higher temperature side higher than or equal to 25° C. as room temperature is an extreme value, and the oscillating frequency varies according to a temperature in any cases of both of the AT-cut and SC-cut crystal elements. For example, in an AT-cut crystal element, the frequency-temperature characteristic shows a cubic curve (curve A in FIG. 5), and in an SC-cut crystal element, the frequency-temperature characteristic shows a quadratic curve (curve B in FIG. 5). Incidentally, frequency deviation Δf/f is plotted along the ordinate of the diagram, where f is a frequency at room temperature, and Δf is a frequency difference from the frequency f at room temperature.
The oscillator output circuit 2 is composed of an oscillating stage 2a serving as an oscillator circuit and a buffering stage 2b having a buffer amplifier or the like. The oscillating stage 2a is formed as a Colpitts type circuit having a voltage dividing capacitor (not shown) and transistor for oscillation, that form a resonance circuit along with the crystal unit 1. Here, the oscillating stage 2a is formed as a voltage control type circuit having a voltage-controlled capacitative element 4Cv in an oscillatory loop, for example. In the drawing, Vcc is a power source, Vout is an output, and Vc is a control voltage.
In the temperature control circuit 3, a temperature sensing voltage Vt by a temperature sensing element (for example, thermistor) 4th and a resistor 4r1 is applied to one input terminal of an operational amplifier 40A, and a reference voltage Vr by resistors 4r2 and 4r3 is applied to the other input terminal. Then, a differential voltage between the reference voltage Vr and the temperature sensing voltage Vt is applied to the base of a power transistor 4Tr, and electric power from the power source Vcc is supplied to the chip resistors (hereinafter called heating resistors) 4h serving as heating elements. In this example, the number of heating resistors 4h (4h1, 4h2) is two. Thereby, the supply current to the heating resistors 4h is controlled with a temperature-dependent resistance value of the temperature sensing element 4th, and a quantity of heat from the power transistor 4Tr as well is added thereto to keep the operational temperature of the crystal unit 1 constant.
The first and second circuit substrates 5a and 5b are both made of epoxy materials, and are formed into a multistage (two-stage) structure in which the outer circumferential portion of the first circuit substrate 5a is supported on the second circuit substrate 5b with metal pins 7, and their board faces face each other. The metal pins 7 electrically connect circuit patterns (not shown) on the first and second circuit substrates 5a and 5b. The heating resistors 4h1 and 4h2, the power transistor 4Tr, and the temperature sensing element 4th of the temperature control circuit 3 are installed on (firmly fixed to) the central area which is the top face of the first circuit substrate.
Then, the principal surface of the crystal unit 1 (the principal surface of the metal cover 1d) is installed on these elements 4(h1, h2, Tr, and th) so as to interpose heat conducting resin 8 therebetween. In this case, the power transistor 4Tr which is the highest is disposed on the principal surface at the leading end side of the crystal unit 1 (the metal base 1b), and the temperature sensing element 4th is to be between the heating resistors 4h1 and 4h2. The pair of lead wires 1c led out from the metal base 1b of the crystal unit 1 penetrates through to be connected to the first circuit substrate 5a. 
Then, a circuit element 4 for adjustment such as, for example, a trimming capacitor (not shown) is installed on the top face of the first circuit substrate 5a, and the circuit elements 4 of the oscillating stage 2a including the voltage-controlled capacitative element 4Cv having an affect on an oscillating frequency are installed on the top and bottom faces. The second circuit substrate 5b is, for example, a laminated plate having a shielding metal film 9 on its lamination plane, and has mounting terminals 10 on its outer bottom face. Then, the other circuit elements 4 of the buffering stage 2b and the temperature control circuit 3 are installed on the inner bottom face of the second circuit substrate 5b. The oscillator case 6 is formed such that a metal cover 11 is bonded to the second circuit substrate 5b serving as the base as well.
Incidentally, JP-A-2008-28620 also discloses a related art constant-temperature type oscillator.
However, in the constant-temperature type oscillator having the above-described configuration, because not only the heating resistors 4h, but also the power transistor 4Tr as well serves as a heating source, the energy efficiency of the power source is enhanced, meanwhile, the power transistor 4Tr is greater in height than the heating resistors 4h1 and 4h2, which generates a step between those. Accordingly, the thickness of the heat conducting resin 8 interposed between the crystal unit 1 and the heating resistors 4h1 and 4h2 as well is increased, and heat loss through the heat conducting resin 8 is generated, which results in the problem of a lack of efficiency of heat conduction.
Further, with respect to the crystal unit 1, the lead wires 1c led out from the metal base 1b are bent to be connected to the first circuit substrate 5a. Then, the principal surface of the crystal unit 1 is installed (laminated) so as to face the heating resistors 4h so as to include the power transistor 4Tr, which results in the problem that the oscillator is basically increased in height size.
For this reason, as shown in JP-A-2005-341191, for example, a circuit substrate is made of ceramic, and a crystal unit formed for surface mounting is installed on one principal surface of the circuit substrate, and heating resistors facing the crystal unit are installed on the other principal surface. Thereby, the crystal unit and the heating resistors are installed on both principal surfaces of the circuit substrate so as not to be laminated, which reduces the oscillator in height size. Then, because heat from the heating resistors is conducted to the principal surface (the bottom face) of the crystal unit by utilizing the heat conducting property of the ceramic (the circuit substrate), it is possible to heat up the crystal unit.
However, in this case, the ceramic is applied as a heat conducting material, meanwhile, the heat conductivity of ceramic is approximately less than or equal to 1/10 of that of metal, which results in the problem of a lack of efficiency of heat conduction. Then, because the power transistor is not used as a direct heat source, there has been the problem of a decrease in energy efficiency as well.